Mud motors, also known as drilling motors, are used in many oil and gas well drilling operations to supply power in the form of rotational mechanical energy downhole. This rotational mechanical energy can be applied to the drill bit, either for increased rate of penetration or for deviation of a wellbore, as in directional drilling operations. Additionally, or alternatively, mud motors can be used for other operations such as driving an electrical generator to power measuring while drilling (MWD) or logging while drilling (LWD) equipment. Mud motors are powered by the flow of drilling fluid, also known as drilling mud, which is pumped down through the drill pipe and drives the mud motor. Mud motors are substantially similar in construction to progressive cavity pumps, and typically include a power section, in which the flow of drilling fluid causes a helical rotor having a certain number of lobes to eccentrically rotate within a stator having at least one additional lobe. This eccentric rotation is typically converted into concentric rotation by a transmission section, which may include, for example, a constant-velocity joint or other equivalent mechanical arrangement.
An important parameter in the operation of a mud motor is the differential pressure across the power section. It is this differential pressure that determines the torque developed by the motor. More specifically, the operating torque developed by the motor is proportional to the differential pressure. Typically a mud motor will be rated to produce a specified torque, which corresponds to a particular differential pressure. Most mud motors may exceed this rated torque for at least limited periods of time, although this requires application of proportionally higher differential pressures. For example, it is not uncommon for a mud motor to have a stall torque (i.e., the highest torque it can produce) that is approximately 2 to 2.5 times the rated torque. However, the basic construction of mud motors serves to limit the differential pressure that may be applied without damaging the motor.
For example, the stator of the power section is typically lined with an elastomer that allows for sealing between the rotor and stator, which is required for operation of the motor. Excessive differential pressure can cause drilling mud to bypass this seal, thereby subjecting the elastomer material of the stator to failure in such forms as chunking or excessive erosion. In some cases this damage may be immediate and catastrophic, resulting in complete failure of the motor. In other cases, limited damage to the elastomer may result, in which case the motor is still operable but can no longer develop the same stall torque. Additionally, in these cases of limited damage, further operation at higher torque levels that may have previously been non-damaging will become further damaging due to the condition of the stator. To protect the power section from either type of damage, it would be desirable to limit the differential pressure applied across the power section of the motor.
The mud motor transmission section can also be damaged by excessive differential pressure. Like any rotational mechanical system, the transmission system has strength limits that are a function of design, size of components, materials, etc. Because of shock loading, also known as dynamic factor or dynamic load, in many cases the transmission section can experience 2 to 2.5 times the torque load that the power section experiences, i.e., 2 to 2.5 times the torque developed by the power section. Depending on various design constraints such as costs, packaging, etc. it may not be possible or practical to design a transmission section for a particular application that could withstand full dynamic loading at the stall torque of the motor. Thus, in such cases, it would be desirable to limit the differential pressure applied to protect the transmission section from mechanical failure.
Additionally, there are many other conceivable applications in which it is desirable to limit differential pressure in a downhole environment. Such applications need not be limited to the use of mud motors, or even to drilling operations, but could arise in completion, treatment, stimulation, or other wellbore operations. In all of the foregoing and other operations, what is needed in the art is an effective, reliable, and repeatable mechanism for protecting downhole tools or the formation itself from the deleterious effects of high differential pressures.